1. Field
This application relates generally to data communication, and to a low power output stage.
2. Background
Various circuit topologies may be used in an output stage of a communication system. In a typical application a load such as a speaker or an antenna may be driven by an amplifier of a particular class, depending on the requirements of the application. For example, a class-A amplifier may be used in applications that may require relatively good linearity but may not require high-efficiency. Class-C amplifiers, on the other hand, provide much greater efficiency at the cost of higher distortion in the output signal. Another class of amplifier, a class-D amplifier, utilizes a switching architecture to provide high-efficiency and is often used in pulse width modulation-related and sigma delta modulation-related applications.
FIG. 1 illustrates a typical implementation of a class-D amplifier circuit 100 for a digital audio application. In this example, a digital audio input signal 102 is converted to a 1-bit sigma delta modulated stream by a sigma delta modulator 104. A 1-bit digital-to-analog converter 106 converts this bitstream into an analog signal that is, in turn, filtered by a low pass filter 108. A comparator 110 compares the reconstructed analog output of the low pass filter 108 with the output of a sawtooth waveform generator 112. A resulting pulse width modulated signal 114 controls a pair of coupled transistors 116 that alternately drive a speaker 118 depending on the level (high or low) of the pulse width modulated signal 114.
A class-D amplifier circuit such as the one depicted in FIG. 1 may provide a relatively efficient output stage because only one of the transistors 116 is on at a given time. In practice, however, the power consumed by the components that generate the pulse width modulated signal 114 (e.g., components 110 and 112) may be relatively significant, particularly in low power applications.
In some applications a sigma delta modulated signal may be used to directly drive the output stage (e.g., the transistors 116). Such a circuit may reproduce the original signal in a relatively accurate manner if appropriate filtering is provided either before or at the output stage. For example, such a circuit may utilize a filter (e.g., a capacitor) prior to the output stage or the load (e.g., a speaker coil) may provide a sufficient filtering effect.